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Willis Renewable Energy Systems / Making it easier to retrofit solar water heating

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Innovation is at the heart of sustainability. Yet often that innovation is about enhancing what we have already got.

The Solasyphon is a perfect example. A plumbing innovation that speeds up and simplifies the retro-fitting of solar water heating in existing homes, it saves the cost and carbon impact of buying a new water tank. So far 1,300 Solasyphons have been sold and there is strong interest from companies as far afield as the US and New Zealand.

Between £600 and £1,200 saved on installation costs.

2,500 Solasyphons installed so far.

Systems sold to date save 600 to 1,300 tonnes/year CO2.

"We’ve had the Solasyphon in place for three years now, and it’s been absolutely fantastic. We have piping hot water all day long. It starts working as soon as the sun hits the solar panel. It’s brilliant in the winter as it activates with the winter sun as so we’re not dependent on a really hot sunny day."

Liz McElkerney, owner of a Solasyphon


Solar water heating is an effective of way of incorporating renewable energy technology at a household level in the UK. When installed during construction of a house, it is straightforward to include a suitably designed hot water tank. However, most hot water tanks in the UK’s 26 million existing homes are not suitable for direct connection to a solar water heating system and it is usually necessary to install a new water tank during a retro-fit.

Willis Renewable Energy Systems has invented the Solasyphon, which allows existing water tanks to be left in place and used with solar water heating. The Solasyphon is just the latest in a series of inventions from Willis, which has been an innovator in plumbing and heating for over 100 years.

A solar water heating installer with a Solasyphon, from Willis Renewable Energy Systems.

The organisation

Willis Renewable Energy Systems is a subsidiary of Willis Heating and Plumbing Ltd, a family business that has been in operation since 1887, has an annual turnover of £1.6m, and employs 20 people. The subsidiary was set up in 2007 specifically to develop and market the Solasyphon. The parent business also installs underfloor heating and ground source heat pumps.

The technology

How does it work?

Solar water heating is normally installed with a specifically designed hot water tank, which includes a heat exchanger near the bottom. The hot fluid from the solar collector panel passes through the heat exchanger, giving up its heat to the water in the tank and then returning to the solar panel. This is the most efficient way of capturing the heat, but the water temperature increases slowly because the whole tank is being heated. Retrofitting solar water heating to an existing house normally requires a new hot water tank, which adds to the cost and disruption to the household during fitting, as well as taking up valuable storage space.

The benefits for me as an installer are there’s much less work. It’s very simple to fit. You just attach [the Solasyphon] to the original heating system and don’t lose any storage space. After 10 or 15 minutes they’re already producing hot water.

Willis installer

The Solasyphon is a heat exchanger installed outside the hot water tank, which allows the existing tank to be retained. It works on the basis that hot water is less dense than cold water, and so rises or ‘thermosyphons’. As the hot water rises out of the Solasyphon it enters the top of the water tank, and cold water is drawn in from the bottom of the tank. As a result, small amounts of hot water are delivered quickly. In standard European tests where hot water is drawn from the tank during the ‘solar day’, the Solasyphon produces a higher average water temperature than conventional technology. Delivering hotter water inevitably leads to a small reduction in the overall efficiency of the solar water heater system.

How much does it cost and how do users pay?

US$1 = £0.81 (Great Britain Pounds) [May 2010]

The cost of a Solasyphon depends on the number of units bought, but it is usually under £300. By using a Solasyphon, the customer avoids the cost of purchasing and installing a new water tank. The net saving is typically £600 for a vented system (equivalent to 15% of the total cost of the retrofitted solar water heating system) and £1,200 for a pressurised system (28% of the total cost).

The Solasyphon is simple to install and saves space.

How is it manufactured, promoted and maintained?

The metalwork of the Solasyphon is manufactured by a local coppersmith business, which also makes water tanks, pipes and other parts for plumbing. The completed assembly is then tested to 10 bar (double the normal mains water pressure) and sent to Willis, where it undergoes a quality check and has an insulation jacket added before the product is packaged up ready for sale. No maintenance is required: the Solasyphon is supplied with a 25 year warranty and is approved by the Water Regulations Advisory Service.


Since production began in 2007, Willis has sold over 2,500 Solasyphons, and is currently selling about 1,000 per year. 300 of these have been installed by Willis itself, with the remainder sold to other installers.

A customer washing dishes in hot water provided by her newly installed Solasyphon system. 

Environmental benefits

The amount of CO2 saved by a solar water heating system depends on a number of factors, including the amount of water used, the size of the solar panel and the energy source it replaces. Typical savings per installation range between 0.26 and 0.58 tonnes/year CO2. Using a Solasyphon allows installations to go ahead where the cost and disruption of replacing the hot water tank would have been a barrier. If all 2,500 Solasyphons sold to date represent installations that would not have happened otherwise, they equate to savings of between 600 and 1,300 tonnes/year CO2 (taking into account the small efficiency penalty of the Solasyphon).

An additional benefit of using a Solasyphon is the saving in the use of copper or stainless steel by avoiding the need for a new hot water tank, and the embedded energy associated with the tank.

Social benefits

By reducing the cost and disruption associated with installing solar water heating, the Solasyphon allows more households to take advantage of renewable energy, saving money on their fuel bills at the same time as reducing CO2 emissions. The cost saving on installation also reduces the payback period of this technology.

It’s a completely home grown product, right through from invention to manufacture. Every part is manufactured here and as production increases the opportunities for creating new jobs will increase.

John Willis

The Solasyphon heats a small quantity of water quickly, bringing considerable benefit to users who frequently need small amounts of hot water throughout the day, potentially reducing the need for an additional heating system to be brought into use.

Willis has worked with local colleges to help them learn first-hand about the latest solar water heating technology and how it is installed, providing them with free Solasyphons for apprentice training. It has also worked with the University of Ulster on research and independent testing of the Solasyphon.

Economic and employment benefits

The Solasyphon has resulted in the creation of business development jobs at Willis, and jobs at their manufacturer. It has also helped solar water heating installers by allowing them to offer a lower cost service and complete the installations more quickly.

Paul, a plumber and installer for Willis Renewables, carries the Solasyphon into the customer’s house.

Potential for growth and replication

The potential for the Solasyphon in the UK and further afield is significant, as there are millions of homes that could be retro-fitted with solar water heating: reducing the cost and disruption of installation is likely to increase take-up of the technology.

A key change for the business in 2009 was the acceptance of the Solasyphon into the Microgeneration Certification Scheme, which enables customers to access grants for installation. The Renewable Heat Incentive, due to start in 2011, will pay customers for the heat generated by their solar thermal systems, further encouraging take-up of the technology.

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